I use an auto transformer, mine is step up and down. If you decide to go the auto transformer route a few tips I can offer:
Good to assume that unless you get a datasheet saying otherwise an auto transformer will need to be rated 2-3 times above the max power of the device. Obviously you can do your own tests, but just putting it out there. e.g.
http://www.sevenstar.com/transformer-guide.htmMine is Seven Star branded and "rated" at 3000W, I wouldn't recommend this brand (though it may be better than others). The socket contacts where really cheap (soldered) and needed tightening, I also tested and traced out the wiring and importantly earthing to make sure it was safe. That said, it was dirt cheap secondhand and does the job.
Not sure what device you're powering but some have a selectable input like mine, it may allow you to tweak between 220V-240V. I wanted the option of between 110V-120V output, so I'm running mine a little "hot" to get a nice 120V. The device I'm currently powering would be lucky to draw 100W but I brought it to cover most situations.
If you want as small as possible go for (or make) one that has the bare transformer with end covers. A transformer in a box increases the size substantially.